Effect of uniform electric field on homogeneous vapor-liquid nucleation and phase equilibria. II. Extended simple point charge model waterWe study model square-well fluids with well-width parameter ϭ1.5 confined to hard planar slits. We derive a general computer simulation method for numerically calculating an arbitrary first derivative of the canonical ensemble partition function with respect to a simulation parameter, which we call the virtual-parameter-variation method. Two special cases of this approach are the Widom test-particle insertion method for calculating the excess chemical potential, and a method for calculating the pressure due to Eppenga and Frenkel ͓Mol. Phys. 52, 52, 1303 ͑1984͔͒. We use this approach to calculate the volume derivative parallel to the slit walls of the Helmholtz free energy in an (N,V,T) Monte Carlo simulation, and show that this spreading pressure is numerically consistent with the thermodynamic pressure obtained by integration of the Gibbs-Duhem equation using the simulated chemical potentials of the confined fluid as a function of density. We obtain new simulation results for the spreading pressure and the phase equilibrium properties of the confined square-well fluid, and we also estimate its critical point properties, observing a decrease of the critical temperature in comparison to the bulk fluid.

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We study the simulation cell size dependence of chemical potential isotherms in subcritical square-well fluids by means of series of canonical ensemble Monte Carlo simulations with increasing numbers of particles, for both three-dimensional bulk systems and two-dimensional planar layers, using Widom-like particle insertion methods. By estimating the corresponding vapor/liquid coexistence densities using a Maxwell-like equal area rule for the subcritical chemical potential isotherms, we are able to study the influence of system size not only on chemical potentials but also on the coexistence properties. The chemical potential versus density isotherms show van der Waals-like loops in the subcritical vapor/liquid coexistence range that exhibit distinct finite size effects for both two- and three-dimensional fluids. Generally, in agreement with recent findings for related studies of Lennard-Jones fluids, the loops shrink with increasing number of particles. In contrast to the subcritical isotherms themselves, the equilibrium vapor/liquid densities show only a weak system size dependence and agree quantitatively with the best-known literature values for three-dimensional fluids. This allows our approach to be used to accurately predict the phase coexistence properties. Our resulting phase equilibrium results for two-dimensional square-well fluids are new. Knowledge concerning finite size effects of square-well systems is important not only for the simulation of thermodynamic properties of simple fluids, but also for the simulation of models of more complex fluids (such as aqueous or polymer fluids) involving square-well interactions.

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